Physical vapor deposition in a vacuum environment is the principal means of depositing thin organic material films as used in small molecule OLED devices. Such methods are well known. It is necessary to know the deposition rate of material to control the final thickness of the desired layer. Vapor deposition systems have often relied upon crystal quartz monitors to determine the deposition rate of a material being vaporized. The vapor condenses on a vibrating quartz element, changing the mass and stiffness of the vibrating element. This disturbance is converted into a signal proportional to the thickness of the condensed material deposited on the vibrating element. These sensors can provide reliable deposition rate and deposited thickness information when the film deposited on their surface is very thin, adheres well to the surface, and has a high density, but their accuracy diminishes rapidly as the deposited film thickness increases. This problem is especially troublesome with low-density films such as with organic materials and has necessitated frequent replacement of the vibrating element and disruption of the manufacturing process. At low deposition rates, the disruption in the manufacturing process has been minimized through the use of a turreted sensor assembly that rotates a new sensor into position when the active element has reached the end of its useful accumulation range. Shutters have also been used to extend the lifetime of a crystal quartz monitor, but no deposition rate information is collected when the shutter is closed, and the accumulated film thickness information is lost. At high deposition rates, both of these solutions are inadequate since the vaporized material reaches a critical thickness on the monitor in a short period of time.
Since the deposition rate of a vaporized organic material from a manifold will depend upon the pressure of the vaporized organic material within the manifold, it is possible to measure the pressure and relate it to the deposition rate. One well-known method of determining the pressure of a gas under vacuum conditions is a Pirani gauge, which measures the heat lost by an electrically heated wire in the gas. Since the thermal conductivity of a gas varies linearly with pressure over a range of pressures, the rate at which the wire loses heat to its surroundings is a function of the gas pressure, and can thus be calibrated to the pressure of the gas present. The wire in a Pirani gauge is typically the variable resistance element in one arm of a Wheatstone bridge circuit. The gauge can be operated in constant temperature, constant voltage, or constant current mode. In the constant temperature mode, the power required to keep the wire at a constant temperature varies with changes in gas pressure, and thus the power supplied acts as a measure of the vacuum. In the constant current and constant voltage modes, the voltage or current respectively acts as a measure of the pressure. In these two modes, the temperature of the wire decreases as the pressure increases. This has the undesirable effect of decreasing the sensitivity of the pressure sensor at higher pressures.
However, standard Pirani gauges have several difficulties in measuring the pressures necessary to achieve the thin films of organic materials desired in an OLED device. Standard Pirani gauges are not always sufficiently sensitive to the pressures required for such coatings when the gauge is heated above the condensation temperature of the organic vapor. To improve the sensitivity of the gauge, it is often necessary to raise the temperature of the wire far above the temperatures of the surroundings. For example, the sensing wire in some commercial Pirani gauges are to be run near 1700° C. Temperatures this high are completely unsuitable for measuring organic materials, which will often decompose at this temperature, leading to loss of costly material, as well as carbon deposition onto the incipient device and the coating apparatus.
A need therefore exists for a way of measuring the deposition rate in vapor deposition systems that overcomes the limitations of the prior art.